test: Add Vitest test infrastructure

[skid-dev]

Summary

Sets up Vitest with WXT's WxtVitest() plugin so unit tests can run against the real WXT config and use fakeBrowser for chrome.* APIs. Uses happy-dom for DOM globals (added because the WXT plugin alone doesn't provide one).

• vitest.config.ts registers WxtVitest() and sets test.environment = "happy-dom".
• New devDeps: vitest@^3.0.0, @vitest/coverage-v8@^3.0.0, happy-dom@^15.0.0.
• New npm scripts: test, test:watch, test:coverage.
• README gains a ### Testing subsection documenting the commands.

Smoke tests

• src/background/set_setting.test.ts — seeds chrome.storage.sync via fakeBrowser, verifies the target key updates without dropping siblings, and verifies the uninitialized-storage path logs Settings not initialized yet. without touching storage.
• src/content/auto_detection/show_banner.test.ts — mounts the banner into document.body, asserts id/top/right/zIndex/position and innerHTML content, then uses vi.useFakeTimers() + vi.advanceTimersByTime(10000) to assert it removes itself. A third case exercises a custom auto_hide_ms.

Test plan

• npm install --ignore-scripts succeeds
• npx wxt prepare succeeds
• npm run typecheck passes
• npm run test — 5 tests pass across 2 files
• npm run build still succeeds (no regression to the chrome-mv3 output)
• Files pass prettier --check against the project's .prettierrc

Notes for the reviewer

• Targets claude/recursing-goldberg-5b5636 (the WXT migration branch from Migrate build system from Webpack to WXT #22). Once that merges to main, this PR can be retargeted.
• The task spec said set_setting should "error out" when storage isn't initialized. The actual implementation logs to console.error and returns — the test verifies that observable behavior rather than asserting a thrown error.

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